A number of hydrocarbons, especially lower-boiling light hydrocarbons, in formation fluids or natural gas are known to form hydrates in conjunction with the water present in the system under a variety of conditions—particularly at a combination of lower temperature and higher pressure. The hydrates usually exist in solid forms that are essentially insoluble in the fluid itself. As a result, any solids in a formation or natural gas fluid are at least a nuisance for production, handling and transport of these fluids. It is not uncommon for hydrate solids (or crystals) to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shutdown, loss of production and risk of explosion or unintended release of hydrocarbons into the environment either on-land or off-shore. Accordingly, hydrocarbon hydrates have been of substantial interest as well as concern to many industries, particularly the petroleum and natural gas industries.
Hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally is composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it is possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulfide are known to form hydrates under the proper conditions.
There are two broad chemical techniques to overcome or control the hydrocarbon hydrate flow hazards, namely thermodynamic and kinetic. The thermodynamic approach is to prevent hydrate formation by addition of “antifreeze” to the production fluids. The kinetic approach generally attempts (a) to prevent the smaller hydrocarbon hydrate crystals from agglomerating into larger ones (known in the industry as an anti-agglomerate and abbreviated AA) and/or (b) to inhibit, retard and/or prevent initial hydrocarbon hydrate crystal nucleation; and/or crystal growth (known in the industry as a kinetic hydrate inhibitor and abbreviated KHI). Thermodynamic and kinetic hydrate control methods may be used in conjunction.
Kinetic efforts to control hydrates have included the use of different materials as inhibitors. For instance, onium compounds with at least four carbon substituents are used as AA to inhibit the plugging of conduits by gas hydrates. Additives such as polymers with lactam rings have also been employed as KHI to control clathrate hydrates in fluid systems. All these kinetic inhibitors are commonly labeled as Low Dosage Hydrate Inhibitors (LDHI) in the art. KHIs and even LDHIs are relatively expensive materials, and it is always advantageous to determine ways of lowering the usage levels of these hydrate inhibitors while maintaining effective hydrate inhibition.
In order to identify and evaluate potential hydrate inhibitors and appropriate dosages or concentrations, a number of bench testing methods have been used, including rocking cells, autoclaves and wheels. One gas hydrate test apparatus includes a bank of pressurized sight glass cells, typically sapphire, each cell containing two stainless steel balls and a pressure transducer. In a typical experiment, each cell is either rocked (to simulate flow conditions) or held static (to simulate a shut-in condition) during the course of each experiment. The rocking motion, when employed, causes the stainless steel balls within the cells to traverse each cell's longitudinal axis, creating additional agitation. During shut-in simulations, the cells are placed at a horizontal stagnant position. Data logging includes monitoring the water bath temperature, the pressure of each cell and periodic visual observations. As all experiments are isochoric, the cell pressure decreases as the cell temperature is lowered. A maximum cell working pressure at room temperature may be about 1500 psig (10.3 MPa). A typical maximum test pressure at 40° F. (4.4° C.) may be between about 1100 psig (7.6 MPa) and 1350 psig (9.3 MPa).
Visual observations include documenting a rating assessment of the cell's contents: a determination if hydrates are visible, an evaluation of any visible hydrate's surface adhesion properties, and an estimate of liquid levels. A “pass” is typically rated as an “A”, “B” or “C”. A “pass” is judged when no hydrates form, or if hydrates do form, the crystals remain small (usually barely visible), the crystals do not agglomerate, the crystals do not adhere to any surfaces, and/or fluid viscosities remain low. A “fail” rating is given (“D” or “F”) if the hydrates form plugs or deposits and/or fluid viscosities increase significantly.
Quality control for KHIs is achieved by performing controlled blank experiments with no hydrate inhibitor present in the system. Moreover, a statistical analysis on induction times of repeat runs also provides a measure of variation. From time to time it has been found that these methods are not very repeatable or consistent, especially when multiphase occurring in the system. The best minimum relative standard deviation (RSD) for rocking cells and autoclave methods is at least 25.
A similar experience has also been encountered in autoclave testing. Autoclave testing involves a high pressure stirred cell that is also filled with brine, gas and condensate or oil. The autoclave may have a sight glass. The cell is placed in a jacket or immersed in a temperature controlled bath. The cell is slowly cooled or quench cooled and held at either constant pressure or constant volume. The inner temperature is monitored, and the contents and viscosity are visually monitored, such as by image monitoring, and the torque on the stirrer is measured.
Wheel apparatus, such as wheel-shaped pipe flow loops licensed by Sintef, may consist of 2- to 5-inch (5.1- to 12.7 cm) pipe, shaped as a circular loop or wheel of 2 meters in diameter that rotates about a horizontal axis. The speed that the wheel rotates determines the flow regime. Peripheral velocities may range from 0.3 to 5 m/s. No pumps or compressors are used. Pressures up to 250 bar (25 MPa) may be applied. The wheel may have at least one high pressure window, often two, that is observed with a video camera. As with the other methods, the wheel is filled with brine, gas and condensate or oil. The wheel is placed in a temperature-controlled chamber. The temperature may range from −10 to 90° C. The pressure, temperature, visual appearance and torque may all be monitored.
Conventional flow loops often comprise a stainless steel loop, usually with a sight glass for image recording. A pump provides circulation and the speed of the pump controls the flow regime. Sizes of flow loops range from bench scales of 0.5 inches (1.3 cm) in diameter to 10 feet long (3 m) up to pilot scales of 4 inches (10.2 cm) in diameter and 275 feet (84 m) long. The flow loops may be filled with brine, gas and condensate or oil. They may be closed systems or at constant pressure. The loop is typically placed in a temperature-controlled chamber or bath. Monitoring is done of the visual appearance, the temperature, the pressure, the pressure drop and/or water conversion. Flow loop reactor testing results are very repeatable. However, even the bench scale experimental apparatus are quite expensive. Each experimental run is typically very time consuming, requires considerable amounts of fluid, and a long lead time. In situations where evaluation of an approach or method to inhibit gas hydrates must occur quickly with limited field fluids, such apparatus are at a disadvantage.
Thus, it is desirable that new apparatus and methods of forming gas hydrate and its inhibition be found which would yield predictable and reliable test results and provide a consistent method for hydrate formation and inhibition testing. Such apparatus and method may ideally be relatively low cost, be easy to set up, be conducted in a relatively short time, and require minimum amounts of fluids.